Grand Targhee

000
FXUS65 KRIW 221800
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1200 PM MDT Sat Aug 22 2026

.KEY MESSAGES...

- Continued rain shower activity extending southwest to
  northeast through the remainder of the afternoon and evening.
  Storm activity possible across eastern counties late
  afternoon, and also Yellowstone towards sunset. Severe weather
  is not likely due to cloud cover at this time. 

- The chance of showers and storms lingers through much of the
  week, although today looks the most active.

- Temperatures will gradually decrease to near normal levels for
  much of the week.

&&

.UPDATE...
Issued at 1200 PM MDT Sat Aug 22 2026

No real changes to the short term forecast. Rain shower activity
been near spot on in relation to HiRes data, especially the
trusty NAMNest as per usual. Storm activity has been limited to
about nothing due to the continued cloud cover depicted on IR. 
As a result, severe weather is looking less likely for eastern 
counties after 4PM. However, storm development looks to still 
occur through 8PM this evening forming a line that will produce 
some stronger outflows up to 50 mph (50-60%). Otherwise, 
instability just won't be there for much, if any, hail. No 
flooding issues with this progressive pattern. Storms again push
out of northeastern Idaho into Yellowstone towards sunset for a
couple hours towards midnight before exiting into Montana. Time
of day and continued cloud cover limit the instability and any 
stronger storms. More isolated than scattered for Sunday 
compared to Saturday. Temperatures for Saturday afternoon will 
be bumped a bit down not taking into account the cloud cover 
limiting solar radiation, but a bit more sun for Sunday. Long 
term forecast is what it is, no changes at this time. 

&&

.DISCUSSION...
Issued at 141 AM MDT Sat Aug 22 2026

The monsoon has once again arrived across the area, I believe this 
is the third or fourth appearance so far. So far, it has behaved. We 
do have some light showers across mainly the western half of the 
area but nothing heavy so far. The main concern in the forecast will 
be the potential for heavy rain and possible strong thunderstorms 
today. Right now, in this meteorologists opinion, potential is mixed 
for if we will see widespread of either.

In regards to heavy rain, the main factor will be the amount of 
moisture in the atmosphere. Dew points have been slow to rise so 
far, but they should this afternoon, rising into the 50s in portions 
of the Big Horn Basin as well as Johnson and Natrona Counties. 
Precipitable water values should also be rather high. The GFS, for 
example, has precipitable water values about 175 to 200 percent of 
normal. This had been overdone by around 15 to 20 percent the past 
few events though. Even with this, precipitable water values over 
150 percent of normal look fairly likely. There will also be some 
forcing in the form of a shortwave moving in from the west. This 
should enhance chances for all areas, so everyone will have a 
chance. The best chance still looks to be in northwestern Wyoming, 
generally a 1 in 2 chance, and there is decent model agreement in 
this. As for severe thunderstorms, there looks to be a decent amount 
of instability. This is especially so in the favored portions of our 
area, mainly East of the Bighorn Range in Johnson and Natrona 
Counties. Lifted indices fall as low as minus 4 to minus 5 with 
close to 1500 J/Kg of surface CAPE here. There is also a decent 
amount of directional shear in this area. This, plus the ample 
moisture, may increase the hail threat in this area.  We do have a 
marginal risk of severe weather in portions of Johnson and Natrona 
Counties, and this could be expanded westward with the 13Z update. 
All areas could have a thunderstorm but with less instability and 
shear most storms would be sub severe.

However, there are some factors against possible hazards. In regards 
to flooding and thunderstorms, it is cloud cover. Much of the area, 
especially western Wyoming, is covered by clouds as I write this 
around midnight. If it stays mostly cloudy for much of the day, this 
could limit instability somewhat and decrease the chance of stronger 
thunderstorms. There will likely be some areas that will be sunnier 
though, especially further east. And there is one more mitigating 
factor for heavy rain and possible flooding. With the shortwave 
moving through, there will be some steering flow today. It won't be 
strong, but most areas will have 15 to 25 knots at 500 millibars 
which should keep storms moving somewhat. We continue to have a 
marginal risk of excessive rainfall, and persistent storms could 
still cause flooding. But it may decrease the chance somewhat. The 
shortwave should clear the area by midnight, but some showers will 
likely linger through the night. As for temperatures, more cloud 
cover should lower them around 5 to 8 degrees, but it will still be 
above normal for almost all areas.

The forecast from Sunday through much of next week looks fairly 
similar. We will remain on the edge of the ridge. Some monsoonal 
moisture will stick around, with precipitable water values remaining 
above average through much of the period. However, they do look 
lower than today, so the chance of convection should be somewhat 
lower.  Most guidance favors western Wyoming at this time. However, 
given the poor performance of the models in regards to timing and 
placement of the showers and storms, confidence in the details 
remains low. Temperatures will also cool off somewhat, back down to 
near to just slightly above normal levels for late August. There 
continue to be some indications of drier air moving in for the end 
of next week or next weekend, possibly shutting off the monsoon and 
bring a period of warm, but not really hot weather for the end of 
August.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1114 AM MDT Sat Aug 22 2026

VFR conditions prevail through the period. With increasing moisture 
across the area, all sites will have a chance of a shower or 
thunderstorm today between 19z and 03z, though some lingering 
showers may occur through about 06z. Gusty outflow winds 30 to 50 
knots will be the main concern with the stronger storms, with brief 
MVFR or IFR conditions if a storm moves directly over. As with usual 
summer afternoon convection, placement is difficult to pinpoint, so 
PROB30 groups cover the chances today. Least chances (about 15%) at 
KLND and KRIW.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for 
the latest information on icing and turbulence forecasts.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

UPDATE...Lowe
DISCUSSION...Hattings
AVIATION...Wittmann